1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the same, and more particularly to a semi-transmissive liquid crystal display device and a method of manufacturing the same.
2. Description of Related Art
Conventionally, two types of liquid crystal display devices, a reflective liquid crystal display device and a transmissive liquid crystal display device, have been known. The former has a reflector therein, which reflects light entered from outside, and utilizes the light as a light source for display, eliminating the need for a backlight as a light source. The latter has a backlight therein as a light source.
The reflective liquid crystal display device provides an advantage over the transmissive liquid crystal display device in that the reflective one consumes lower electric power and is fabricated thinner and lighter. Accordingly, the reflective liquid crystal display device is utilized primarily as a portable terminal device. This is because the reflective liquid crystal display device utilizes light as a light source for display so that light entered from outside is reflected by a reflector provided within the device, eliminating the need for a backlight. The transmissive liquid crystal display device is able to display an image to be displayed, with visibility enhanced to a larger extent than that observed in the reflective liquid crystal display device when the surroundings are dark.
The current liquid crystal display device basically comprises a liquid crystal layer which is of one of twisted nematic (TN) crystal type, one sheet polarizer type, super twisted nematic (STN) crystal type, guest host (GH) type, polymer-dispersed liquid crystal (PDLC) type, cholesteric liquid crystal type and the like, a switching element for driving a liquid crystal cell, and a reflector provided inside or outside the liquid crystal cell or a backlight. The liquid crystal display device constructed as described above generally employs an active-matrix drive scheme to achieve high precision and high image quality by using as a switching element a thin film transistor (TFT) or a metal-insulator-metal (MIM) diode. The liquid crystal display device further includes a reflector or a backlight.
A semi-transmissive liquid crystal display device having both advantages observed respectively in the reflective and the transmissive liquid crystal display devices is disclosed in Japanese Patent No. 2955277 and configured as shown in FIG. 1. A gate wiring 2 and a drain wiring 3 are formed intersecting one another in directions orthogonal to each other and disposed along peripheries of a pixel electrode 1 on a TFT substrate (hereinafter, a substrate having a thin film transistor thereon is referred to as a TFT substrate). In this case, a thin film transistor 4 is assigned to the pixel electrode 1, and the gate wiring 2 and the drain wiring 3 are connected to a gate electrode and a drain electrode of the thin film transistor 4 respectively. In the pixel electrode 1 are formed a reflective region 5 (indicated by cross-hatch) made of a metal film and a transmissive region 6 made of an Indium-Tin-Oxide (ITO) film.
As described above, forming the transmissive region and the reflective region in the pixel electrode makes it possible to use a liquid crystal display device with a backlight turned off as a reflective liquid crystal display device when the surroundings are bright, thereby effecting low power consumption for a reflective liquid crystal display device. Furthermore, in a case where a liquid crystal display device is used as a transmissive liquid crystal display device when the surroundings are dark and the backlight is turned on, the liquid crystal display device enhances the visibility of an image to be displayed when the surroundings are dark, which operation is featured in a transmissive liquid crystal display device. Hereinafter, a liquid crystal display device used as both a reflective and a transmissive liquid crystal display devices is referred to as a semi-transmissive liquid crystal display device.
However, light travels different distances in a liquid crystal layer respectively when transmitting through a transmissive region 6, and when being incident on and reflected by a reflective region 5. Accordingly, the above-described regions have retardation values different from one another, causing the problem of impossibility of optimizing the intensity of light exiting from the device. To solve the problem, a liquid crystal display device disclosed in Japan Patent No. 2955277 is configured to have a cross section shown in FIG. 2. As shown in FIG. 2, the liquid crystal display device is configured to have an insulation layer 8 formed below a transparent electrode 7 of the reflective region 5 and dispose a reflector 9 above or below the insulation layer 8, causing a difference between a film thickness dr of a liquid crystal layer in the reflective region 5 and a film thickness df of a liquid crystal layer in the transmissive region 6.
FIG. 3 illustrates a graph indicating the result obtained by calculating the intensity of light exiting from the device and measured in a transmissive mode and a reflective mode based on a thickness of a liquid crystal layer when a twist angle Φ is equal to 0 degree. The graph indicates that the intensity of light exiting from the device and measured in a transmissive mode and a reflective mode varies depending on a thickness of a liquid crystal layer. The graph also indicates that both behaviors for the intensity of light traveling in a transmissive mode and a reflective mode are made nearly equal to each other when a ratio of the film thickness dr of a liquid crystal layer in the reflective region 5 to the film thickness df of a liquid crystal layer in the transmissive region 6 is set about 1:2 to eliminate a difference between distances which light travels through a liquid crystal layer in the reflective region 5 and the transmissive region 6. The inventors of this application further studied to optimize the intensity of light exiting from the reflective region and the transmissive region of the semi-transmissive liquid crystal display device constructed as described above. The results obtained by the study will be shown below.
(1) Optimization of the Intensity of Light Exiting from the Reflective Region and the Transmissive Region.
FIG. 4 is a diagram illustrating how light transmitting through the corresponding portions of a semi-transmissive liquid crystal display device is polarized. FIG. 5 is a diagram illustrating a relationship between the thickness of a liquid crystal layer and the twist angle of liquid crystal molecules. The semi-transmissive liquid crystal display device shown in FIG. 4 is assumed to have a reflective electrode 10 disposed on an insulation layer 8 shown in FIG. 2 and serving as a reflector.
As shown in FIG. 4, the semi-transmissive liquid crystal display device comprises a TFT substrate 11, an opposing substrate 12, a liquid crystal layer 13 interposed between the above-mentioned substrates, a backlight disposed below the TFT substrate 11 in the device, optical compensators 120, 220 and polarizers 123, 223 provided at corresponding outer positions of the TFT substrate 11 and the opposing substrate 12.
(Disposition of an Upper Polarzer and an Upper λ/4 Plate)
The optical compensator (λ/4 plate) 220 is disposed between the liquid crystal layer 13 and the polarizer 223 to establish a normally-white mode in the reflective region, which mode is defined such that a voltage is not applied between the opposing substrate and both the reflective region and the transmissive region to make liquid crystal molecules lie in parallel with the surfaces of the substrates and display “white,” and a voltage is applied therebetween to make liquid crystal molecules rise up and display “black.” The λ/4 plate 220 is made to rotate 45 degree, relative to an optical axis of the polarizer 223 and then interposed between the polarizer 223 and the liquid crystal layer 13, which configuration makes linearly (horizontally) polarized light transmitting through the polarizer 223 become right circularly polarized light after transmission through the λ/4 plate 220. The right circularly polarized light reaches the reflective electrode 10 maintaining itself as a linearly polarized light by setting a specific value for the film thickness dr of the liquid crystal layer in the reflective region. The linearly polarized light is reflected as it is by the reflective electrode 10 and becomes right circularly polarized light when exiting from the liquid crystal layer 10. The right circularly polarized light is made linearly (horizontally) polarized light by the λ/4 plate 220 and exits through the polarizer 223 with an optical axis parallel to a horizontal direction to the outside, thereby displaying a white color.
On the other hand, when a voltage is applied to the liquid crystal layer 13, liquid crystal molecules rise up. In this case, light entering the liquid crystal layer 13 as right circularly polarized light reaches the reflective electrode 10 maintaining itself as right circularly polarized light and is reflected as left circularly polarized light by the reflective electrode 10. Then, the left circularly polarized light exiting as it is from the liquid crystal layer 13 is converted to linearly (vertically) polarized light by the λ/4 plate 220 and does not exit from the device because the light is absorbed by the polarizer 223, thereby displaying a black color.
(Disposition of a Lower λ/4 Plate and a Lower Polarzer)
In a case where the liquid crystal display device is in a transmissive mode, an angular relationship between optical axes of the lower λ/4 plate 120 and the lower polarzer 123 is determined to display a black color during application of voltage to the liquid crystal layer. The lower polarizer 123 is disposed in relation to the upper polarizer 223 so that the two polarizers constitute crossed Nicol prisms, i.e., being disposed to rotate 90 degree with respect to the upper polarizer. Furthermore, to eliminate (compensate for) the influence of the upper λ/4 plate 220, the lower λ/4 plate 120 also is disposed to rotate 90 degree with respect to the upper one. Since the liquid crystal molecules are being rising up during application of voltage thereto and polarized light does not change its polarized state, the polarizers 123, 223 are disposed in a state optically equivalent to the configuration of crossed Nicol prisms, making light transmitting through the liquid crystal layer display a black color during application of voltage to the liquid crystal layer. Thus, the disposition of optical components constituting a semi-transmissive liquid crystal display device and the angular relationship between optical axes of the optical components are determined.
When the optical components are disposed maintaining the above-described angular relationship and the twist angle Φ of liquid crystal molecules is made to vary from 0 degree to 90 degree, the optimal film thickness dr of a liquid crystal layer in the reflective region 5 for making the reflectance to display a white color maximized and the optimal film thickness df of a liquid crystal layer in the transmissive region 6 for making transmittance to display a white color maximized are measured and shown in FIG. 5. As shown in FIG. 5, the optimal film thickness dr of a liquid crystal layer in the reflective region 5 and the optimal film thickness df of a liquid crystal layer in the transmissive region 6 coincide with each other at a twist angle=72 degree and the optimal film thickness dr in the reflective region becomes smaller than the optimal film thickness in the transmissive region in proportion to the decrease in the twist angle of liquid crystal molecules. For example, in a case where a nematic liquid crystal having a birefringence of 0.086 (Δn=0.086) is employed to forma liquid crystal, when a twist angle is set at 72 degree, the optimal film thickness df of a liquid crystal layer in the transmissive region and the optimal film thickness dr of a liquid crystal layer in the reflective region each are 2.7 μm (micrometers) and when a twist angle is set at 0 degree, the optimal film thickness df of a liquid crystal layer in the transmissive region is 2.9 μm and the optimal film thickness dr of a liquid crystal layer in the reflective region is 1.5 μm.
(2) Condition for Effectively Reflecting Light in a Direction Normal to the Surface of Reflector.
FIG. 6A schematically illustrates how incident light Li incident on the reflector 32 is reflected as reflected light Lr by the same and the light Lr is viewed by a viewer. Assume that an angle between the incident light Li and the direction normal to the surface of reflector and an angle between the reflected light Lr and the direction normal to the same are respectively referred to an incident angle Ti and a reflected angle Tr. Since the incident light Li is reflected by a reflective electrode 35 formed to have a concave-convex profile that follows the profile of a later described projection pattern 33 and second insulation film 34 shown n FIG. 8B, the incident angle Ti and the reflected angle Tr are different from one another.
FIG. 6B is a diagram schematically illustrating how light incident on a point A of the reflective electrode 35 having a concave-convex profile is reflected by the same and showing only a surface profile of the reflective electrode 35 and the reflector 32 for simplicity.
When the incident light Li enters the point A of the reflective electrode 35 having a concave-convex profile, the incident light Li is reflected at the point A by the tangent plane, meaning that the incident light Li is reflected as the reflected light Lr in a direction symmetrical to the normal to the tangent plane at the point A.
It should be noted that when assuming an angle, formed at the point A, between the tangent plane at the point A of the reflective electrode 35 and the reflector 32 is defined as a tilted angle .theta., distribution of reflected directions of the reflected light Lr varies depending on the distribution of the tilted angle .theta. at which the concave-convex of the reflective electrode 35 is tilted to a surface of the reflector 32. This indicates that based on estimation made by a viewer P who subjectively evaluates a brightness given by the reflector 32, a designer needs to design the reflective electrode so that the viewer P is able to feel the reflected light bright when viewing the reflective electrode that is formed reflecting distribution of the tilted angle .theta. at which the surface of the reflective electrode 35 is tilted to a surface of the reflector 32.
It would primarily appear that the viewer P views a reflective liquid crystal display device or a semi-transmissive liquid crystal display device under the following environment. As shown in FIG. 7A, the viewer P views reflected light Lr in a direction ranging from −10 degree to +20 degree relative to the direction normal to the reflector 32 after the incident light Li from a light source S disposed ranging from 0 degree to −60 degree is relative to the same is reflected by the reflector 32. As shown in FIG. 7B, the viewer P views reflected light Lr in a direction ranging from −20 degree to +20 degree relative to the direction normal to the point A of the reflector 32 after incident light Li from a direction ranging from −20 degree to +20 degree is reflected by the reflector 32.
Making many concave-convex portions, which extends in a horizontal direction when viewed by the viewer P, included in the concave-convex pattern formed in the reflector 32 allows a designer to design the reflector 32 having a directivity of reflection, which directivity is observed when the incident light Li from the light source S is effectively reflected as the reflected light Lr toward the viewer P.
FIG. 8A is a plan view of the concave-convex pattern formed in the reflector 32. A cross-hatch portion in the figure is a region in which a convex pattern 33 is formed and a region indicated by open triangles is a region in which a concave portion is formed. As shown in FIG. 8A, although triangles indicating the concave portion are arrayed in an orderly fashion, the triangles actually are disposed in a fairly random fashion. Although a liquid crystal display device is exemplified in which three sides of each of multiple triangles are defined by the convex pattern 33, a liquid crystal display device may be exemplified in which the convex is patterned to define rectangles or ellipses (enclosed figures) enclosed by a linear convex pattern to thereby form a concave-convex pattern.
FIG. 8B is a schematic cross sectional view taken along line X–X′ shown in FIG. 8A. Assume that a distance between centers of the linear portions of the convex pattern 33 in a width direction, those linear portions being determined so as to interpose a rectangle therebetween while substantially penetrating the center of the triangle, is L, a width of the convex pattern 33 is W, a height of the convex pattern 33 is D, a minimum height of a second insulation film 34 is “d,” a difference between a maximum height of the second insulation film 34 and the minimum height thereof is ΔD representing a height of the step of the concave-convex of the reflective electrode. Since the film thickness of an aluminum film (the reflective electrode 35) coated on an upper surface of the second insulation film 34 is thin, the aluminum film is drawn as a line in the figure for simplicity as a result of ignorance of thickness of the aluminum film.
Referring to FIG. 7A, it is required to determine the surface profile of the reflective electrode to increase reflectance in a reflected angle ranging from 0 degree to 10 degree. The surface profile of the reflective electrode is approximately determined by ΔD representing a height of the step of the concave-convex of the reflective electrode 35 and the a distance between centers of the linear portions of the convex pattern 33 (a first insulation film) in a width direction, both being shown in FIG. 8B.
Recently, a liquid crystal display device has been required to display an image with high precision. In addition, a semi-transmissive liquid crystal display device is becoming the prevalent display device employed in a portable equipment such as a portable telephone, reflecting the need for a bright screen of display device. As a liquid crystal display device provides a higher precision image and is supplied more frequently as a semi-transmissive one, the number of triangles (concave portions) included within a pixel decreases. Accordingly, a problem arises in that reflected lights interfere with each other. This is because decreasing the number of triangles included in a pixel makes it difficult to eliminate interference between reflected lights within the pixel. For this reason, the distance L between centers of the linear portions of the convex pattern (first insulation film) 33 in a width direction needs to be as small as possible. However, currently the distance L is forcibly made ranging from 60 to 80 μm since the distance is determined by manufacturing capability such as exposure accuracy. As a result, the surface profile of the reflective electrode is approximately determined by the ΔD representing a height of the step of the concave-convex of the reflective electrode 35.
What the market mainly demands from a semi-transmissive liquid crystal display device is displaying a bright image and in order to display the bright image, the liquid crystal display device has to satisfy the following conditions: (1) in accordance with the twist angle of liquid crystal molecules, the optimal film thickness dr of a liquid crystal layer in the reflective region 5 for making the reflectance to display a white color maximized and the optimal film thickness df of a liquid crystal layer in the transmissive region 6 for making transmittance to display a white color maximized need to be determined as shown in FIG. 5; (2) the optimal surface profile of the reflective electrode needs to be determined so as to effectively reflect lights incident thereon in a direction normal to the reflector.